1. Field of the Invention
The present invention relates to a method and its apparatus for measuring the displacement of an object using light interference, and more particularly to a method and its apparatus for measuring the displacement amount of an object from an interference signal obtained by applying a laser beam to the object and causing interference between the reflected light and a reference light.
2. Description of Related Art
A method for measuring the displacement amount or movement amount of an object using light interference is widely known. For example, in an AFM (Atomic Force Microscope) that is one kind of an SPM (Scanning Probe Microscope) known as a measuring tool of fine solid shape, a displacement sensor using light interference has been adopted as a displacement sensor for measuring the position of a probe with means capable of measuring the smaller displacement at high sensitivity, instead of the conventional capacitance sensor, as disclosed in semiconductor MIRAI project lithography related instrumentation technology workshop (Oct. 19, 2004) document, p. 28 (2004). In this light interference displacement sensor, a laser beam from a laser light source is divided into two, in which one of the beams is directed to a reference mirror, and the other is directed to a target mirror attached on a probe scanning mechanism to cause interference between both reflected lights (reference light and measuring light) with a phase difference of 0, π/2, π and 3π/2, and four generated phase-shifting interference lights are received and converted into electrical signals, thereby calculating the movement amount of the probe through a desired computation process for four phase-shifting interference signals.
However, in the above light interference displacement sensor, there was a problem that the overall optical system is larger and the light interference displacement sensor is increased in size to limit the application objects, because as a method for providing a phase difference of 0, π/2, π and 3π/2 between the reference light and the measuring light, it employs a prism for synthesizing the reference light and the measuring light, a non-polarizing beam splitter for splitting the synthesized light into two optical paths, and two polarizing beam splitters for giving a phase shift between the reference light and the measuring light that make up the synthesized light in two optical paths.
Therefore, it was nearly impossible that three light interference displacement sensors for x, y and z axes, which are larger than the conventional capacitance sensor, were mounted on the AFM applied to a mass production line such as a semiconductor manufacturing process. Further, it had an essential problem that there occurs an error in performing a computation process for calculating the movement amount of an object from the four phase-shifting interference signals, if there is a different disturbance such as a temperature distribution, a humidity distribution, an air pressure distribution, a density distribution or an air flow distribution on the four optical paths, because the optical paths for the four interference lights with a phase shift provided are separate away from each other.